1. Field of the Invention
The present invention relates to a liquid crystal composition, a liquid crystal compound and a liquid crystal display device and more in particular, it relates to a tetrahydropyran compound, a liquid crystal composition containing the same and having a nematic phase and a liquid crystal display device containing the liquid crystal composition.
2. Related Art
A liquid crystal display device utilizes optical anisotropy and dielectric anisotropy inherent to a liquid crystal substance. The liquid crystal display device is classified into various types including a TN type (twisted nematic type), a DS type (dynamic scattering type), a guest-host type, a DAP type (deformation of aligned phase type), an STN type (super twisted nematic type) and a TFT type (thin film transistor type), and the TN type, the STN type and the TFT type are becoming mainstream in recent years. It is necessary that a liquid crystal material used in any of these types of display devices has stability to water, air, heat and light, and also it exhibits a liquid crystal phase in a wide temperature range as much as possible centering on room temperature, has a low viscosity, has an optimum dielectric anisotropy value (Δ∈), and has an optimum refractive index anisotropy value (Δn), an optimum elastic constant K and an optimum elastic constant ratio K33/K11 (K33: bend elastic constant, K11: splay elastic constant). It is the current situation that there is no single substance that satisfies these requirements, and several to several dozen kinds of liquid crystal substances are mixed to fulfill the aforementioned required characteristics. Upon preparing a liquid crystal composition, there are some cases where the required characteristics cannot be fulfilled due to a too small Δ∈ and a too large Δn, for example.
The requirements to the characteristics of the liquid crystal composition are currently becoming severe for attaining a liquid crystal display device having high performance. In order to satisfy the requirements, such a liquid crystal compound is demanded that can highly adjust the properties of the liquid crystal composition.
It is currently demanded that a liquid crystal display can be driven at a low temperature. In general, compounds used in a composition having a low Δn contain a large amount of cyclohexane rings, and a smectic phase or crystals are liable to deposit from the composition due to the poor compatibility of the composition at a low temperature. Under the circumstances, development of a liquid crystal compound having a small Δ∈ and a high clear point or a liquid crystal compound having a small Δ∈ and being excellent in compatibility is demanded.
As a liquid crystal compound that are used in the aforementioned purposes, a compound represented by the following formula (16) has been reported in German Patent No. 3,306,960, for example. However, the compound (16) has a large Δn and an insufficient compatibility at a low temperature.

The related art of the invention is disclosed in the following patent documents: EP 1 302 523, DE 19 950 194, DE 19 733 199, DE 19 625 100, GB 2 310 669, DE 19 611 096, WO 9 632 851, DE 4 238 377, DE 4 222 371, WO 9 213 928, DE 4 132 006, WO 9 105 029 and EP 376 294. Further preferred liquid crystal compound, liquid crystal composition and liquid crystal display device are demanded.